94 



is all that remains of this region in the Eutherian brain. All that 

 remains of the corresponding part of the fornix is the striae Lancisii. 

 The fornix commissure forms therefore, in the brain of the placental 

 Mammal, the morphological posterior extremity (which from bending 

 of the hemisphere becomes ventral — the psalterium) of the great 

 supraventricular commissure. The greater part of the olfactory centre 

 of higher Mammals thus comes, by accident, as it were, to be placed 

 posteriorly. But it serves no useful purpose morphologically, nor is 

 it strictly correct, to compare the whole hemisphere of Petromyzon 

 with the posterior part of the higher brain, as Studnicka does 0, on 

 the ground that the former is entirely olfactory. 



In discussing the morphology of the cerebral hemisphere there 

 are two fixed points which may be recognised in all Vertebrates. The 

 situation of the anterior commissure in the lamina infraneuroporica is 

 one of these. The relation of the fornix commissure to this point 

 varies. Thus in many Amphibia, most Sauropsida and the early stages 

 of the Mammal, commissure in question is immediately dorsal to the 

 anterior commissure and the two often appear in median sagittal 

 section to form one band. As development proceeds in the Mammal 

 the two commissures become separated by a grey mass formed from 

 the growing lamina infraneuroporica. This is the Anlage of the sep- 

 tum pellucidum. Since the fornix-commissure [the corpus callosum of 

 BuRCKHARDT ^] is situated in the lamina infraneuroporica, it must be 

 placed at first on the ventral aspect of the recessus neuroporiCus. 

 Thus in the foetal Platypus the fornix fibres descend upon the lateral 

 aspect of the little diverticulum, to cross below it. But as the lamina 

 increases it extends upwards and the fornix commissure grows back- 

 wards above the recessus neuroporicus (lobus olfactorius impar), so 

 that it is quite conceivable that the latter may come to occupy the 

 situation of the recessus triangularis of Schwalbe (the "aulic recess" 

 of Wilder) as Kupffer holds ^), although the fornix commissure and 

 probably the corpus callosum are both constituents of the lamina infra- 

 neuroporica, and therefore morphologically ventral to the recessus 

 neuroporicus. There is certainly no ground for comparing the latter 

 recess with the recessus splenialis ventriculi septi pellucidi, as Burck- 

 HARDT has done. Such a comparison is opposed to all the facts of 

 the development of the corpus callosum. This further statement that 



1) Anat. Anz., Bd. X, No. 3. 4. 



2) Morphologische Arbeiten, Bd. IV. 



3) Studien zur vergleichenden Entwickelungsgeschichte des Kopfes 



